Transfer elements having microporous ink layers containing pressure-exudable flowable ink are well-known, and reference is made to U.S. Pat. Nos. 3,037,879 and 3,689,301 as representative thereof.
Such transfer elements were originally developed for typewriter use in place of conventional wax-base carbon papers since the microporous ink layer did not transfer under a single impact pressure, as is the case with one-time wax carbon layers, but rather functioned by exuding pressure-flowable ink from the microporous sponge binder material each time the transfer element was subjected to typing pressure, even over the same area.
More recently, it has been found desirable to use such transfer elements in ribbon form in high speed typing or impact printing machines and bar code printers which operate at such speeds that conventional one-time transfer elements are impractical since they are used up so quickly that they must be replaced at frequent intervals. The microporous transfer ribbons may be reversed and reused several times, depending upon the printing machine, or more commonly, are transported in a continuous creeping motion through the machine so that each area of the ribbon is subjected to several overlapping impacts before it passes the impact station as the machine transfers characters at a rate of up to twenty per second --see Xerox U.S. Pat. Nos. 3,954,163 and 4,037,706, for example.
An important problem has been encountered during the use of such microporous transfer ribbons in high speed printing machines and bar code printers in which the ribbon is in constant motion during impact, i.e., it is hit by the type element or print face without any stoppage of movement of the ribbon --see U.S. Pat. No. 3,924,532. The copy sheet may also be in movement in the same direction or in a direction perpendicular to the direction of movement of the ribbon. In any event, the ribbon is moving slowly through the impact station when it is engaged by the typing face and, even though the period of engagement may be momentary, the typing face embosses and can snag the rear face of the ribbon and cause a breakdown of the bond between the film foundation and the ink layer supported thereby. This breakdown results in a transfer of solid particles of the microporous resin structure of the ink layer to the copy sheet, rather than the desired exudation of the ink from the microporous resin structure, and the production of spotty and dirty images on the copy sheet.
Also, particularly in the case of narrow ribbons, the frictional engagement or snagging of the moving ribbon by the type face can result in a breakage of the ribbon.
It was felt that such problems were inevitable with the use of microporous transfer ribbons in high-speed impact and printing machines, due to the spongy nature of the ink-exuding microporous resinous layer and the high pressures exerted by the type element, print hammer or bar code fonts.